Water and/or other condensible gases, such as CO.sub.2, are common impurities in many raw or process gases. These impurities are known to cause, among other things, unwanted reaction and corrosion in various systems which employ gases. To inhibit these detrimental effects, gases are usually pretreated to remove the impurities. Some of the conventional techniques used to remove the impurities, herein referred to as "wet components", include compression, cooling, adsorption and membrane separation. Among these conventional techniques, the membrane separation may be most attractive for economically dehydrating or drying wet gas streams, particularly in small scale operations.
The membrane separation generally involves the selective permeation of the wet components in a feed gas stream through utilizing a membrane module or cartridge. The membrane module or cartridge typically comprises membrane materials in the form of a plurality of small hollow fibers, which are disposed within an enclosure. The hollow membrane fibers, which may be constructed with synthetic Polymers or inorganic materials, are usually arranged to provide a large membrane surface area with particular flow configurations so that the wet components of a feed gas can selectively permeate therethrough in an efficient manner. The wet components of a feed gas, of course, can be selectively permeated from or to either side of the large membrane surface, i.e., from the outside, or shell side, of the hollow fibers to the fiber bores or from the fiber bores to the outside, or shell side, of the hollow fibers, as long as a pressure difference is maintained across the membrane. The wet components in a feed gas at the permeation pressure, for instance, permeates through the membrane to reach the lower pressure, permeate side thereof. The partial pressure difference that drives permeation will, however, be significantly reduced unless the wet components permeated to the low pressure, permeate side of the membrane are removed to maintain a low partial pressure of the components on the permeate side. Once the saturation point of the permeate is reached, then other undesirable effects such as capillary condensation could occur which could further reduce the permeation flow of the wet components.
It has been known to utilize a gas which is being dried to reduce the wet components vapor pressure on the permeate side of a membrane to below the saturation point. The membrane module sold under the tradename "Prism Cactus" by Permea Inc., Malvern, Industrial Park, Box 396, Malvern, Pa. 19355, for example, is designed to permeate a particular amount of the gas being dried together with the wet components. The permeating gas, in turn, carries an adequate amount of the wet components away from the low pressure, permeate side of the membrane so that the wet components vapor pressure on that side can be maintained below the saturation point. This permeation technique, however, is found to be inefficient not only because a higher pressure difference is needed to permeate a gas which is less permeable than the wet components, but also because the amount of the gas permeated for carrying out this purpose is substantial and represents loss of product.
In order to minimize these inefficiencies, the use of at least a portion of the dry product resulting from the membrane drying or a sufficiently dry gas from an external source to sweep or purge the wet components on the low pressure, permeate side of a membrane has been proposed. U.S. Pat. No. 4,931,070--Prasad and U.S. patent application Ser. No. 07/596,098, now U.S. Pat. No. 5,084,073--Prasad, for example, disclose a membrane module having at least four ports, one of which being used to introduce a recycling portion of the dry product gas or a gas from an external source to the low pressure, permeate side of a membrane to flush the wet components which are permeating through the membrane. As a result of using the dry product gas or an external source gas as purging means, the amount of the desired product gas lost to the permeate side of a membrane is decreased. Moreover, there is no need for a high pressure difference across the membrane since no gas need be permeated through the membrane with the wet components. In spite of these improvements, however, there remain certain limitations. For example, a large amount of the product gas must still be lost as a purge waste stream when the product gas is used to purge the wet components. If a sufficiently dry external source gas is substituted for the product gas as a purging means, the product gas can become contaminated due to back permeation of some of the external source purge gas across the drying membrane. It is, therefore, desirable in the art to develop improved membrane drying processes and systems to reduce the amount of the product gas lost to the permeate side of the membrane as well as the contamination level in the product gas.
It is an advantage of the present invention in providing an improved permeation drying process and system wherein the amount of dry product gas required for purging is reduced.
It is an additional advantage of the present invention in reducing the level of contamination due to back permeation when a foreign dry gas is employed as the purge means.
It is a further advantage of the present invention in providing an efficient and economical permeation process and system for removing water vapor from high purity nitrogen, which has been treated in a "Deoxo" process to remove traces of oxygen, with negligible recontamination of the product nitrogen during the drying process.